In recent years, an increase in an efficiency of a signal amplifier in a transmitter of a wireless communication device is required. A class-D amplifier using switching operation is a useful amplifier whose amplification efficiency is theoretically 100 percent. As a transmitter using such a class-D amplifier, mentioned are a configuration using IF-DSM (Intermediate Frequency-Delta Sigma Modulation) and that using EDSM (Envelope Delta Sigma Modulation). A configuration of a transmitter using IF-DSM is described, for example, in non-patent document 1 (A. Frappe, B. Stefanelli, A. Flament, A. Kaiser and A. Cathelin, “A digital Δ-Σ RF signal generator for mobile communication transmitters in 90 nm CMOS”, IEEE RFIC Symp., pp. 13-16, June 2008). A configuration of a transmitter using EDSM is described, for example, in non-patent document 2 (Y. Wang, “An improved Kahn transmitter architecture based on delta-sigma modulation”, IEEE Microwave Symposium Digest, Vol. 2, pp. 1327-1330, June, 2003) and non-patent document 3 (Dupuy A. and Y. Wang, “High efficiency power transmitter based on envelope delta-sigma modulation (EDSM)”, IEEE Vehicular Technology Conference, Vol. 3, pp. 2092-2095, September, 2004).
FIG. 1 is a block diagram showing an example of the configuration of the transmitter using IF-DSM. The transmitter performs Δ-Σ (delta-sigma) modulation of an orthogonal radio signal (IQ radio signal) generated at a digital baseband, and subsequently generates an RF (Radio Frequency) band radio signal by means of digital IQ modulation.
FIG. 2 is a block diagram showing an example of the configuration of the transmitter 100 using EDSM. The transmitter generates an amplitude signal and a phase signal separately from an IQ radio signal generated at a digital baseband, and subsequently, after performing Δ-Σ modulation of the amplitude signal, generates an RF-band radio signal by multiplying the modulated amplitude signal by the phase signal. Specifically, first, a signal to be modulated is split into upper and lower branches by a splitter 103. In the upper branch, an envelope detector 101 extracts amplitude information from the signal to be modulated, and inputs it to a Δ-Σ modulator 102. The Δ-Σ modulator 102 converts the amplitude information into a rectangular signal. On the other hand, in the lower branch, a comparator 104 generates a phase signal by processing the signal to be modulated into a rectangular signal. The phase signal undergoes delay adjustment by a delay adjuster 105 so that its timing is coincident with that of an output signal from the Δ-Σ modulator 102 in the upper branch. The rectangular signal including amplitude information outputted from the upper branch and the rectangle signal including phase information outputted from the lower branch after the delay adjustment are mixed by a mixer 106, and thus are converted into a rectangular radio signal. Here, the rectangular radio signal includes, in addition to a desired signal, quantization noise arising in the process for obtaining a rectangular signal. The rectangular radio signal is amplified by a signal amplifier 107, subsequently undergoes suppression of its unnecessary quantization noise by an output filter 108, and finally becomes a transmission signal radiated from an antenna. Here, the signal amplifier 107 is assumed to be an amplifier with switching operation such as a class-D one, but may also be connected with a signal amplifier without switching operation such as class-A or class-AB one.
Hereinafter, description will be given for the Δ-Σ modulator. Although the Δ-Σ modulator may have various circuit configurations depending on its order and the like, a first-order Δ-Σ modulator with 1-bit output is taken as an example in the following description. FIG. 3 is a circuit configuration diagram of the first-order Δ-Σ modulator. If the z-transforms of input and output signals of the Δ-Σ modulator are represented by X(z) and Y(z), and that of the quantization noise arising in a quantizer by N(z), the following equation stands.
                              Y          ⁡                      (            z            )                          =                                            α                              1                +                                                      (                                          αβ                      -                      1                                        )                                    ⁢                                      z                                          -                      1                                                                                            ⁢                          X              ⁡                              (                z                )                                              +                                                    1                -                                  z                                      -                    1                                                                              1                +                                                      (                                          αβ                      -                      1                                        )                                    ⁢                                      z                                          -                      1                                                                                            ⁢                          N              ⁡                              (                z                )                                                                        (                  equation          ⁢                                          ⁢          1                )            
In the right hand side of the equation 1, a coefficient of X (z) is a signal transfer function, and that of N (z) is a noise transfer function. As the equation 1 shows, the signal transfer function and the noise transfer function depend on multiplication coefficients, α and β of a multiplier. Although it is not shown in the equation 1, the noise transfer function is also influenced by parameters of the quantizer, which will be described below.
The Δ-Σ modulator starts operation by being triggered by a rising edge or a decaying edge, or the both, in a signal in toggle operation. Here, a temporal interval between adjacent edges triggering the Δ-Σ modulator operation is not limited to a constant period, and may be inconstant.
An adder performs addition or subtraction between two pieces of input data. The multiplier multiplies input data by the multiplication coefficients (α, β). A delay device delays the input data by a time period equivalent to a single one of the above-described interval between adjacent edges in a signal in the toggle operation triggering the Δ-Σ modulator operation. From the delaying operation, it can be referred to also as a storage element storing a single piece of input data. The quantizer quantizes input data with reference to a threshold value. The quantizer has two kinds of parameters: a threshold value and output values.
FIGS. 4(a)-(c) show relational diagrams between a threshold value and input-output values of the quantizer constituting the Δ-Σ modulator. As shown in FIG. 4 (a), in the case the output of the quantizer has two levels, with reference to a single threshold value (0, for example), input values are classified corresponding to two output values (+1 and −1, for example). In this case, the output value becomes +1, if an input value of the quantizer is equal to or larger than 0, and becomes −1, if an input value is negative.
As shown in FIG. 4 (b), in the case the output of the quantizer has three levels, with reference to two threshold values (+0.5 and −0.5, for example), input values are classified corresponding to three output values (+1, 0 and −1, for example). In this case, the output value becomes +1, if an input value of the quantizer is equal to or larger than +0.5, becomes 0, if an input value is equal to or larger than −0.5 and smaller than +0.5, and becomes −1, if an input value is smaller than −0.5.
As shown in FIG. 4 (c), in the case the output of the quantizer has four levels, with reference to three threshold values (+0.25, 0 and −0.25, for example), input values are classified corresponding to four output values (+1, 0.5, −0.5 and −1, for example). In this case, the output value becomes +1, if an input value of the quantizer is equal to or larger than +0.25, becomes +0.5, if an input value is equal to or larger than 0 and smaller than +0.25, becomes −0.5, if an input value is equal to or larger than −0.25 and smaller than 0, and becomes −1, if an input value is smaller than −0.25.
Further, signal and noise transfer functions of the Δ-Σ modulator are influenced by, in addition to the parameters of the quantizer (threshold values and output values) and multiplication coefficients (α, β), the order of the Δ-Σ modulator. The transfer functions are closely related to ACPR (Adjacent Channel leakage Power Ratio), which is one of essential standards concerning radio characteristics of a transmitter.
Specifically, a shape in shaping of the quantization noise that determines ACPR is dependent on the order and multiplication coefficients of a Δ-Σ modulator and the number of output bits of the quantizer. Generally, if the order increases, the degree of freedom of the shape in the shaping increases, and noise in the vicinity of a transmission signal band can thus be reduced. Further, if the number of the bits increases, electrical power of the whole quantization noise can be reduced. That is, by increasing the order or the number of the bits, the ACPR can be improved resultantly.